(a) Technical Field
The present invention relates to a fuel cell separator and a method for surface treatment of the same. More particularly, it relates to a fuel cell separator and a method for surface treatment of the same, in which ionized nanoparticles are attached to the surface of a separator to form fine projections such that the surface of the separator exhibits superhydrophobicity.
(b) Background Art
A typical polymer electrolyte membrane fuel cell (PEMFC) can generate electricity with heat and water by an electrochemical reaction between hydrogen and oxygen as reactant gases. The PEMFC has various advantages such as high energy efficiency, high current density, high power density, short start-up time, and rapid response to a load change as compared to the other types of fuel cells. For these reasons, it can be used in various applications such as a power source for zero-emission vehicles, an independent power plant, a portable power source, a military power source, etc.
The configuration of a fuel cell stack will be briefly described below. In a typical fuel cell stack, a membrane-electrode assembly (MEA) is positioned in the center of each unit cell of the fuel cell stack. The MEA comprises a polymer electrolyte membrane, through which hydrogen ions (protons) are transported, and an electrode/catalyst layer such as an air electrode (cathode) and a fuel electrode (anode), in which an electrochemical reaction between hydrogen and oxygen takes place, disposed on each of both sides of the polymer electrolyte membrane.
Moreover, a gas diffusion layer (GDL) and a gasket are sequentially stacked on both sides of the MEA, where the cathode and the anode are located. A separator including flow fields for supplying fuel and discharging water produced by the reaction is located on the outside of the GDL, and an end plate for supporting the above-described components is connected to the outermost ends.
Therefore, at the anode of the fuel cell stack, hydrogen is dissociated into hydrogen ions (protons, H+) and electrons (e−) by an oxidation reaction of hydrogen. The hydrogen ions and electrons are transmitted to the cathode through the electrolyte membrane and the separator, respectively. At the cathode, water is produced by the electrochemical reaction in which the hydrogen ions and electrons transmitted from the anode and the oxygen in air participate and, at the same time, electrical energy is produced by the flow of electrons is supplied to a load requiring the electrical energy through a current collector of an end plate.
Meanwhile, the separator has a land as a flat portion being in direct contact with the GDL and a channel 24 as a space between the lands, through which hydrogen and air (oxygen) passes. The separator functions to supply reactant gases such as hydrogen and air, remove water produced at the cathode from the GDL, and transmit electricity to an external circuit.
The types of the separators are divided into a graphite separator formed by a mechanical process and a metal separator formed by a stamping process. The channel may have a rectangular cross-section or a trapezoidal cross-section. The surface of each channel is smooth, and thus the actual flow of reactant gases has laminar flow characteristics.
However, laminar flow is a flow where the flow velocity at the wall is smaller than that at the center of the channel, and thus a smooth film flow of water droplets cannot be ensured. This makes it difficult to remove the water trapped in pores of the GDL.
Various methods for solving these problems have been proposed, including Korean Patent Application Publication No. 2010-0088346, U.S. Pat. No. 7,250,189, U.S. Patent Publication No. 2010/0035091, U.S. Pat. No. 6,730,238, etc.
First, FIG. 4 refers to Korean Patent Application Publication No. 2010-0088346 discloses a method for forming a CrN—CH thin film on a metal separator for a proton exchange membrane fuel cell and a product thereof. Specifically, a pretreatment process, controlled by circuit board 110, supplies a reactant gas, via gas input device 420, to the inside of a chamber 400 to pre-treat the surface of a substrate 100, and an arc discharge process, executed by an arch device 430, supplies a reactant gas to the inside of the chamber 400 to generate an arc discharge and, at the same time, collects electrons of a Cr target induced by the arc discharge in an anode 440 to accelerate the decomposition of reactants in the chamber 400. Furthermore, a coating process feeds a carbon compound and N2 as reactants into the chamber and, at the same time, applies a bias voltage to the substrate 100 such that the reactants decomposed by the arc discharge are adsorbed to the surface of the substrate 100 and metal ions emitted from the Cr target are contained in the substrate, thus forming a conductive and corrosion resistant thin film on the substrate. Furthermore, a shielding membrane 435 is provided in between the arc device 430 and the substrate 100. For example, the shielding membrane 435, fixed onto the center of the upper end of chamber 400, is transported in an up-and-down motion, or is rotatable horizontally.
According to this method, a corrosion resistant conductive film having high coating uniformity is formed on the separator by vacuum plasma. However, a wetting angle of water droplets is an intermediate angle of 90 to 110, and thus it is difficult to remove the water droplets flowing through channels. Moreover, it requires an expensive vacuum chamber and high purity reactant gas supply equipment, and the output and quality of the separators may be limited according to the size of the vacuum chamber.
U.S. Pat. No. 7,250,189 discloses an electroconductive porous substrate such as carbon fibers with an electroconductive polymer deposited on the carbon fibers, in which the polymer is deposited from a solution of monomers by electrochemical polymerization. However, the electrochemical polymerization is sensitive to the concentration of monomers in the substrate and the solution of monomers, and thus it is difficult to obtain uniform coating on the substrate.
U.S. Patent Application Publication No. 2010/0035091 and U.S. Pat. No. 6,730,238 disclose methods for pre-treating a carbon substrate having a coating with active groups. In particular, a polymer, including ionic or polar groups, is grafted onto the substrate by reaction with the active groups, and polymer functional groups or hydrophilic functional groups are attached to the surface of the substrate. However, the coating is formed depending on the types of gases used in the processes, and thus the types of usable gases are limited. Moreover, in the case of a metal separator having a thickness of 0.1 mm or less, there may be defects such as holes when corrosion occurs due to plasma.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.